1. Field of the Invention
The present invention relates to interposers for integrated circuit chips and more particularly to a method for fabricating a chip interposer, starting with a substrate featuring many chip locations, to be subsequently diced into a plurality of chip interposers, which can each carry one or more chips.
2. Description of the Prior Art
It is well known that integrated circuit chips may be mounted onto a multilayer ceramic substrate (MLC) through small solder ball joints (known as C-4 joints), which are used to bond the chips to pads on the substrate.
In today's technology, the MLC substrate performs a number of electrical functions: it provides voltage planes, signal lines, ground planes and redistribution planes. Redistribution planes are a set of planes at the top part of the module, that translate the internal, wide, via grid of the MLC module, to the much tighter grid for the chip connections. The redistribution planes are the most difficult to make, since they contain a very dense via grid and very dense wiring. The redistribution planes are also a source of unwanted signal noise.
Future chips will not only require more connections between the chip and the MLC, but will also require connections on a denser grid than today's chips. This requirement makes it difficult to make MLC economically and it exacerbates the electrical noise problem, and causes yield problems since redistribution planes of grater complexity will be needed.
These problems could be alleviated, by placing the redistribution on top of the MLC using fine line wiring, also known as TFR (thin film redistribution). The fine line wiring consists of ground planes and wiring planes. The connection with the chips is made on top of the TFR, with the aid of terminal pads, which connect via holes at the upper surface of an MLC substrate. TFR depends on photolithography to make the patterns of features. TFR construction is detailed in various U.S. patents assigned to the assignee of the present invention, see for example, U.S. Pat. Nos. 3,726,002 and 3,968,193.
The MLC substrate, which will incorporate TFR, includes a multi-level metallization at a plurality of planes. Further, vertical conductive paths through the MLC layers forming the substrate are provided by a plurality of metal-filled vias.
The TFR structure basically comprises metallurgy located at different levels separated by a plurality of insulating (e.g. polyimide or glass) layers. Vertical interconnections between different levels of metallurgy are provided through via holes. In one implementation, the first level of metal is Chromium-copper-chromium (Cr-Cu-Cr) while the second level of metal is copper-chromium (Cu-Cr). An additional land of metal may be formed to permit the bonding of the chip solder balls.
Future substrates with TFR need to carry several chips. The size of the substrate will be in the range of approximately 6.5.times.6.5 cm.sup.2 to approximately 20.times.20 cm.sup.2, typically approximately 10.times.10 cm.sup.2. The wiring requirements for TFR do not permit enough space for redundant lines. In other words, all lines on the TFR must be electrically sound if the substrate is to perform properly. If the substrate plus TFR are not electrically perfect, one has to either discard the (expensive) substrate or strip the TFR and begin the complex process all over again or engage in an equally expensive repair process, all of which are expensive alternatives. The processes required for economically making high yield, defect-free planes of fine line wiring over 10.times.10 cm.sup.2, exceeds the capability of present and foreseen technologies.
To deal with this yield problem, a known approach has been to make the TFR on a simple, inexpensive, piece of ceramic which is separate from the MLC substrate. This ceramic would be built by the conventional techniques of punching, screening and laminating, containing only via connections from the front to the back.
This technique permits one to sort out the good sections of TFR and discard the bad ones. The good sections can then be attached to the MLC substrate.
To make the TFR sections economically (i.e., to reap the benefits of batch fabrication), one has to fabricate the TFR sections on large pieces of ceramic. The limitation to this batch fabrication, is the uncertainty about the precise location of the front-to-back vias. This uncertainty ensues from variations of firing shrinkage from substrate to substrate and within one substrate. As mentioned before, TFR is made with photolithography, which, to attain its fine line wiring capability, relies on precise location of all surface features to which it is to be aligned.
The article entitled "Semiconductor device carrier for Modules" authored by M. T. McMahon, published in the IBM Technical Disclosure Bulletin, Vol. 18, No. 5, pp 1440 and 1441 (ref.1) is a good example of the prior art, in that it proposes an interposer made by essentially the same technology as the main body of the MLC substrate: the interposer is made by casting green sheet, punching, screening the lines and firing. The described interposer does not contain a ground plane, so it cannot compete with TFR-like structure.
The publication mentioned above, uses conventional punching and screening techniques for making the interconnections, the so-called screened vias.
Punching and screening are known to be expensive steps not directly applicable to batch processing. Furthermore, there is an additional disadvantage, specific to single chip interposers, in terms of testability. Probing large areas, such as a multichip interposer which are made by photolithography, is more desirable because the chip positions are exactly in the right location.
As a consequence, multichip interposers or carriers have also been suggested in the prior art; see for example IBM Technical Disclosure Bulletin, Vol. 23, No. 9, Feb. 1981, pp. 4062-4063, an article entitled "Pin/chip Carrier Assembly" by S. M. Jensen et al. (ref.2). However, the multichip interposer which is described in this article is also made according to standard MLC technology. Therefore, most of the drawbacks mentioned above still remain. In particular, if any defect is found in the interposer, the whole part is to be discarded.